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So we've finally got into A/C season up here in CT, and this weekend I run across condensing unit (2 ton?) that the HO says the compressor was changed out 2 yrs ago (not our company), but now, there is no cooling. A guage hookup revealed a dead flat charge....actually a little negative (vacuum) reading on the low side guage!!

So I dump in what I got in the truck for R22, maybe 3# worth. The unit responds well, but high side is close to 300 psi and low side is only 35 psi, so I stop. Superheat, probably 50 degrees, I didn't bother measuring since I didn't have anymore R22 and the head pressure was way too high. Plus, it's a holiday weekend and this HO always calls afterhours/weekends for service. Another tech is going back today, but says those readings are sure to mean a bad compressor....I say, more like the system has air or whatever in it besides R22, driving the high side to 300 psi and still being low on charge.

So what is it? I say compressor is fine, since it can make high head pressure and is steady. Low side is steady too, just too low. Maybe replacement compressor is wrong size if all else is copasetic? Advice?

Originally posted by BaldLoonie Full of air, recover what you put in, pull a deep vac with new f/d and recharge.

That was my exact recommendation to the daytime tech (I only do afterhours HVAC) but he's content on selling a new unit it appears. The HO is a doctor, so all in all, probably the right way to go in this situation.

So what is it? I say compressor is fine, since it can make high head pressure and is steady. Low side is steady too, just too low. Maybe replacement compressor is wrong size if all else is copasetic? Advice?

Seems like a badly oversized compressor would just outright appear out of place inside that unit, giving off the first clue. Piping doesn't match, mounting feet don't match mounting pattern on condenser floor, info on compressor tag doesn't come close to condenser size, etc.

So I lean toward non-condensables. And likely nobody checked out why the last compressor died, so that prior problem may now be compounded, making it a wonder this comp is still alive.
A comp making 300 psig head and pulling 30 suction isn't having a problem making pressure at all. Sounds more like a refrigeration circuit problem, if anything.

This is where you seperate your company from the less than professional,remove and replace the old dryer(there should be one if the compressor was replaced)recover and replace the refrigerant with virgin R-22.Now you are in a position to offer advice to the H.O.Remember that we are only as good as our last visit.Even though it may be best to replace equipment,if we can't explain the advantages to the client,we are seen as greedy and incompetent.

compressor is fine
put in three or more pounds of r-22 now 300/35
possible dirty coil or restriction in lines.
my real concern is where did the freon go the first time
replace a blown compressor then you go out empy freon soon to be another blown compressor. where is the leak. selling a new condenser is not the answer finding the leak and fixing it then putting the unit on line once and for all
is the answer

Find the leak and repair it. Recover the refrigerant. Pull a good vacuume. Clean the coil. Install a new filter dryer, use an HH core just to play it safe. Recharge the unit to factory specs. Put in new air filters, belts, etc. Check over the units operation, motor amps, pulleys, shieves, etc...

UPDATE: another daytime tech serviced the unit and found a leak in the condensor. The Doc said enough is enough and authorized a full change out, so tommorrow a new 3 ton 10 seer Armstrong CU is being installed. I still don't see how the old unit/new compressor made high head and low suction with a leak in the condensor though. I still insist the old unit was full of non-condensables (air, moisture, etc) which would drive the head pressure skyhigh. Low suction, of course, meaning low on charge. THanks for the recommendations.

Find leak an repair, pressure test, pull vac, recharge, if system has a txv hold bulb in hand and watch your gauges, if gauges don't respond, replace txv. If evap has an orfice you might have a restriction.